Green water in your garden pond is caused by an algae with the botanical name Chlamydomonas (algae is actually a diverse group of aquatic organisms). Some are familiar to most of us like, seaweed, pond scum, green water, blanket weed or the algal blooms in lakes.
The two types of algae I'm going to show you how to get rid off, are;
The main cause for both green water algae and blanket weed is high fertility (to much food) from excessive nutrients, mainly from fish poo (especially Koi who seem to spend all their time eating and pooing), uneaten fish food, organic debris (rotting pond plants and falling leaves), fertiliser run-off into the pond from higher ground and fertilisers added to plants you have bought from your garden center, insufficient shade in very sunny sites (algae thrives on sunshine). Some algae is natural in a newly built and filled pond and will clear if you follow the tips below.
An algae attack is normally worse in spring and summer, during sunny weather.
So what's the answer?
The water in my nature pond is clear and apart from a few months after building and filling it with tap water, always has been clear. So what's my answer? It's simple, follow these 10 good pond-keeping tips and don't use chemicals.
The algae that causes green water cannot be filtered out it is a micro-organism. Therefore, it must be starved out.
In Summary there's a variety of things to do and not to do that can help deal with green water, all aimed at starving out the algae that causes it. Mostly natural and easy to do without using chemicals, the methods include avoiding fertiliser, removing decaying matter, not over-stocking with fish, oxygenating and aerating the water, avoiding chemicals, sensible shade planting and keeping a healthy garden pond.